This invention relates to a method for assembling gas turbine engine components and more particularly to assembling turbine sets to increase the usable life of a row of cooled turbine airfoils.
As is well known, gas turbine engines include multiple rows of airfoils and the failure of any single airfoil in a row constitutes a failure of the engine. These airfoils operate in a very harsh temperature environment and the individual airfoil temperatures are controlled by passing relatively cooler air through internal passages in the airfoil and/or applying a thin film of cool air on the external surface of the airfoil. Those skilled in the art will appreciate that the life of an individual airfoil is a function of its temperature history. In general, more cooling flow leads to lower temperatures which in turn leads to higher turbine blade or vane life. Conversely, lower life airfoils typically pass less coolant flow relative to other airfoils in a row. It is therefore desirable to increase the flow through such low-life airfoils while at the same time not adversely affecting the other airfoils in the same row.
The cooling airflow through each individual blade or vane is driven by a pressure ratio across the element. The pressure ratio represents the ratio between the blade supply pressure and the dump pressure. The supply pressure is the pressure just upstream of the entrance to the cooling passages on the bottom of the blade and the dump pressure is the gaspath pressure on the outside of the blade. The physical features of each blade determine the relationship between mass flow through the blade and the pressure ratio across the blade. This relationship of mass flow through the blade and the pressure ratio across the blade is referred to as the “characteristic curve” of the blade.
As with all manufacturing operations, cooled turbine blades are subject to manufacturing variation. This variation comes from a variety of sources such as casting and film hole geometric variability. As a result, each manufactured turbine blade exhibits a slightly different relationship between mass flow and pressure ratio. That is, for a given mass flow of cooling air, each individual turbine blade would operate at a different pressure ratio. Conversely, for a given pressure ratio, each individual blade will have a different mass flow. There is an acceptable tolerance during manufacturing on this pressure ratio and blades that demonstrate pressure ratios above or below set limits are unacceptable. In general, acceptable blades span the range between the low and high pressure ratio limits. High pressure ratio and low pressure ratio blades are often referred to as low-flow and high-flow blades, respectively.
A primary object of the present invention is a method of selecting blades for assembly into turbine rows that can increase the mass flow to low-flow blades thereby increasing the life of the blade row relative to current turbine row assembly practice. Furthermore, the present invention has applications to a wide variety of components in a gas turbine engine in which multiple, supposedly-nominal components with flow passages are assembled in parallel.